Plural beam cathode ray tube having boundary-shaped cylindrical electrodes correcting beam aberration

ABSTRACT

A cathode ray tube comprising an electron gun for producing a plurality of beams located in a flat plane passing through the axis of the electron gun. The electron gun system comprises at least two cylindrical electrodes arranged in consecutive order and surrounding the beams. The ends of the two electrodes confronting each other are shaped to provide an aberration reducing electrostatic field for those electron beams that do not coincide with the axes of the two electrodes.

United States Patent Barton 5] July 18, 1972 [54] PLURAL BEAM CATHODE RAY TUBE [56] References Cited HAVING BOUNDARY-SHAPED CYLINDRICAL ELECTRODES UNTED STATES PATENTS CORRECTING BEAM ABERRATION 2,412,687 12/1946 Klemperer ..313/86 2,957,106 10/1960 Moodey ..313/70C X Inventorl PM Gerard J p Baden, Emmasmgel, 3,435,268 3/1969 Rublack ..313/7o c Netherlands [73] Assignee: U.S. Philips Corporation, New York, NY. Primary Examiner 9 Sega] Att0rneyFrank R. Tnfan [22] Filed: Feb. 5, 1970 211 App]. No.: 8,871 [571 ABSTRACT A cathode ray tube comprising an electron gun for producing [30] Foreign Application Prior"), Data a olurality of beams located in a flat plane passing through the axis of the electron gun The electron gun system comprises at Feb. 8, Netherlands least two cylindrical electrodes arranged in consecutive order and surrounding the beams. The ends of the two electrodes [52] 11.8. C1 5313/69 C, 313/70 C, 3131/82 BF f ti each other are shaped to provide an aberration 1 3*- Cl J 29/50, 31/20, 29/02 reducing electrostatic field for those electron beams that do Fleld of Search .-3 l C, not coincide the axes of the two electrodes 2 Claim, 14 Drawing Figures PATENTED JULI 8 m2 3.678.316

sum 1 BF 4 INVENTOR. PIET 6.J.BARTEN AGENT PATENTEI] JULI 8 I972 sum 3 or 4 fimaaaas fig.8

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INVENTOR. PIET 6.1. BARTEN main PLURAL BEAM CATHODE RAY TUBE HAVING BOUNDARY-SHAPED CYUNDRICAL ELECTRODES CORRECTING BEAM ABERRATION This application is related to applicant's copending patent application Ser. No. 66,072, filed Aug-2i, 197! which is a continuation of applicant's parent application Ser. No. 769,612, filed Oct. 22, i968 and now abandoned.

The invention relates to a cathode ray tube comprising an electron gun system for producing a plurality of electron beams, having at least two common, substantially circularcylindrical and substantially coaxial electrodes whose axes do not coincide with the axis of one or more electron beams and comprising a display screen. The invention particularly relates to such a cathode ray tube comprising a display screen having a plurality of differently luminescing substances and a color selection electrode arranged at a short distance from the display screen, while the electron gun system produces a number of electron beams corresponding with the number of luminescing substances, which beams are caused to converge in the plane of the color selection electrode at least by means of a lens field obtained by means of the common, substantially circularly cylindrical electrodes.

in said color tube each of the electron beams is focused on the display screen or at least on a plane located near said screen, for example that of the color selection electrode. This is performed partly by the lens effect between successive grids of the gun or ofthe guns, which thus provide a prefocusing effeet. The lens effect obtained with the aid of the common, substantially circular-cylindrical electrodes causes the electron beams to converge substantially in the plane of the color selection electrode, for example. a shadow-mask electrode. This lens effect can be obtained by means of a substantially circular-cylindrical grid and a conductive coating on the envelope, with the aid oftwo substantially circular-cylindrical grids having, in a particular case, substantially equal diameters, or with the aid of more than two substantially circular-cylindrical grids. This lens effect provides, in addition, the required postfocusing of each electron beam. Such an electrostatic converging system is particularly employed when the beams are closely adjacent each other, because in this case little space is available for the pole pieces required for magnetic convergence. This may apply to a cathode ray tube in which the various electron beams are produced by a single gun. If in the event of three electron beams the centers of the beams in the gun form the corners of an isosceles triangle, a dynamic convcrgcncc is required. in principle. the dynamic convergence may be dispensed with in the case of a given type of deflection coil, provided the electron beams in the gun are located substantially in one plane.

lt has been found that if the end faces of each of the said substantially circular-cylindrical electrodes located on the side of or within the other electrodes associated with the convergence are arranged in flat planes at right angles to the axis of the cylinder. a rotation-symmetrical field is produced, it is true, but that such a lens field brings about an aberration not being rotation-symmetrical to the axis of the beam in postfocusing a beam whose axis is located outside the axis of the cylinder. As a result the electron spot produced by the beam assumes an astigmatic form. The rays of the beam located in the plane containing the axis of the beam and the axis of the cylinder are focused towards the axis of the beam to a stronger extent as a result of the spherical aberration of the lens than the rays located in the plane containing the axis of the beam and being perpendicular to the first plane. A plane going through the axis of the cylinder is termed a meridional plane of the lens and the said plane going through the axis of the beam and the axis of the cylinder is therefore the meridional plane of the lens containing the axis of the beam. The rays of the beam located in said plane are termed the meridional rays. A plane at right angles to the meridional planes of the lens is termed the sagittal plane of the lens and the rays contained in such a plane are termed the sagittal rays. The rays contained in said plane going through the axis ofthe beam and being perbeam axis have to be considered to be sagittal rays. During post-focusing of a beam whose axis is located substantially on the cylinder axis said focusing difference does not occur, since at this place a field rotation-symmetrical to the beam axis is available.

The invention is based on the recognition of the fact that the difference in post-focusing of the meridional rays and the sagittal rays of a beam whose axis is located outside the cylinder axis may be reduced by a defined disturbance of the rotation-symmetry. However, it has to be provided that if the axis of a beam coincides with the axis of the cylinder the field desired for said beam is available at the area of the cylinder axis in spite of said disturbance of the rotation-symmetry. in a given case the difference in post-focusing of the meridional rays and the sagittal rays of a beam whose axis is located outside the cylinder axis, has to be eliminated and at the place of the cylinder axis the field has to remain substantially rotationsymmetrical. According to the invention the axes of the electron beams and the substantially circular-cylindrical electrodes are located substantially in a flat plane, the axis of one beam substantially coincides with the axes of the substantially circular-cylindrical electrodes and the distances between the line forming an effective boundary line of the substantially circular-cylindrical electrodes and the display screen at the area of the points of intersection with said plane are relative maxima and greater than the average distance and the distances at the area of the points of intersection with the plane perpendicular thereto and going through the axes of the electrodes are relative maxima. The following may clarify the term "effective boundary line of the substantially circular-cylindrical electrode." if the lens effect is obtained with the aid of one substantially circular-cylindrical grid and a conductive coating on the envelope, the diameter of the grid is smaller than the diameter of the relative part of the envelope and the conductive coating on the envelope usually overlaps a part of the grid. The effective boundary line is in this case the line located within the conductive coating and which the end of the grid describes on the cylinder. if the lens effect is obtained by means of two substantially circular-cylindrical grids of unequal diameters and if one grid overlaps the other, the effective boundary line in this case also is the line which, the end of the grid describes on the cylinder of the smaller diameter. if the conductive coating and the grid or the grids do not overlap each other, which is particularly true in the case of two grids having equal diameters, the effective boundary line is the line passing through the centers of the gap formed between the end of one electrode and the projection of the end of the other electrode on the cylinder plane of one electrode. With two grids of equal diameters the effective boundary line is the line passing through the centers of the gap between the grids. The desired effect, as a result of the fact that the distance of the ef fective boundary line from the display screen at the area of the points of intersection with the plane containing the electron beams is a relative maximum and greater than the average distance, is based on the fact that the cross-sections of the equipotential planes with sagittal planes in the area of the axis of a beam located outside of the cylinder axis are curved so that the concave side faces the display screen so that an intensifted focusing of the sagittal rays is produced. whereas the crosssections of the equipotential planes with meridional planes in the area of the beam axis are curved so that the con vex side faces the display screen so that a reduced focusing of the meridional rays is produced. The post-focusing difference of the meridional and sagittsl rays of such a beam is reduced. If desired, the sagittal rays and the meridional rays are substantially focused at one point with the appropriate proportioning. in order to ensure that nevertheless at the area of the cylinder axis the field required for the beam there present is produced, the distances between the line forming an effective boundary line of the substantially circular-cylindrical electrodes and the display screen at the area of the points of intersection with the plane perpendicular to the plane containing pendicular to the meridional plane of the lens containing the the axes of the electron hem and going through the axis of map-m the electrodes are relative maxima. The distance at said areas is particularly substantially equal to the distance at the area of the point of intersection with the plane going through the axes of the electron beams. The strength of the converging lens in two orthogonal planes going through the axis of the cylinder is in this case invariably the same so that the field has four planes of symmetry. which is a favorable situation for the beam whose axis coincides with that of the cylinder.

The effective boundary line may have different shapes. It may have a square-wave form. The boundary line preferably has a flowing shape. because this results in a more gradual variation of the equipotential planes. In the particular case in which the distance between the effective boundary line and the dis lay screen has four equal maxima. the effective boundary line preferably has a sinusoidal shape, because the variation of the equipotential planes is then as gradual as possible.

If the effective boundary line coincides with an end of a rid. this end should show the said variation. If the effective boundary line coincides with a line passing through the centers of the gap as described above. the said variation may be obtained by profiling only one of the ends of the electrodes determining the gap and by arranging the other end in a plane normal to the axis of the electrodes. Both ends may be profiled. as an alternative. namely particularly as complementury profile.

By obviating the astigmatism a more circular electron spot is produced on the display screen and it has furthermore found that the spot is smaller.

If the beams. after having lcfl the prcfocusing lens. are not stigmatic. said shape of the effective boundary line permits of obviating the astigmatism so that also in the case circular electron spots are obtained for all beams on the display screen. It is furthermore possible to produce an elongated, for example. vertical electron spot on the display tube starting from beams of stigmatic nature. This cannot be achieved for all beams. if they pass through a converging system. the circular-cylindrical electrodes of which have cnd faces located in a flat plane since with the beam whose axis coincides with the axes of the electrodes the rotation-symmetry is maintained. while it is not certain whether the correct degree of astigmatism is obtained with the beams located outside the axis. The elongated electron spots can. however. be obtained by the described shape of the effective boundary line. In the two cases the distances between the line forming an effective boundary line of the substantially circular-cylindrical electrodes and the display screen at the and of the points of intersection with the plane normal to the plane containing the axes of the electron beams and passing through the axes of the electrodes are not equal to the distances at the area of the points of intersection with the lane going through the axes of the electron beams. The effective boundary line then has in particular the shape of two supcrimposcd sine lines. the period of one being included twice and the period of the other being included four times in the boundary line. In this way the boundary line has a smooth shape.

The invention will now be described more fully with reference to the drawing. in which FIG. I is a sectional view of a cathode ray tube.

FIG. 2 shows given parts of the sectional view of FIG. 1 on an enlarged scale.

FIG. 3 shows the shape of the converging electrodes not in accordance with the invention.

FIG. 4 is a cross sectional view of the electrodes of FIG. 3.

FIG. 5 is a plan view of the electrodes of FIG. 3.

FIG. 6 is a side elevation of the electrodes of FIG. 3.

FIG. 7 illustrates the path of rays of a beam in the case of FIG. 3 in schematic view.

FIG. 8 shows a shape of the converging electrodes according to the invention.

FIG. 9 is a cross sectional view of the electrodes of FIG. 8.

FIG. 10 is a plan view of the electrodes of FIG. 8.

FIG. II is a side elevation ofthe electrodes of FIG. 8.

FIG. 12 is a developed view of part of FIG. 8.

FIG. 13 a developed view of two circular-cylindrical grids and FIG. 14 is developed view of two circular-cylindrical grids.

The cathode ray tube 1 of FIG. 1 comprises a gun 2 (shown schematically) producing three electron beams. whose centers are located in a plane going through the axis of the gun. while the axis of the central electron beam coincides with the axis of the electron gun. The electron gun 2 converged these three electron beams on a shadow-mask 3. after which the beam strike each given parts of a luminescent screen 4. The screen is scanned by means of a deflection device 5 shown schematically.

FIG. 2 is a cross sectional view of the neck ofthe tube taken on the axis of the gun. The gun comprises three cathodes 6. 7 and 8. a common first grid 9 having openings 10. 11 and 12 for the beams from the cathodes 6. 7 and 8. a common second grid 13 having openinm 14. 15 and 16 and a common third grid 17 having openings 18. 19 and 20. The centers of the openings l1. l5 and 19 are located on the axis 21 of the gun. whereas the centers of the openings 10. 14 and 18 are located on a line 22. parallel to the axis 21 and the centers of the openings 12. I6 and 20 are located on a line 23. also parallel to the axis 21. In this case the line 22 coincides with the axis of the cathode 6. the axis 21 coincides with the axis of the cathode 7. and the line 23 coincides with the axis of the cathode 8. The gun comprises furthermore a common fourth grid 24 and a common. circular-cylindrical fifth grid 25. The fourth grid 24 comprises two interconnected. circular-cylindrical portions 26 and 27. Only the sectional lines are shown for the grids 24 and 25. The prefocusing of each of the electron beams is performed between the second grid 13 and the fourth grid 24. whereas port-focusing is performed in the converging lens. which is an accelerating lens comprising the fourth grid 24 and the fifth grid 25. the lens field being produced between the circular-cylindrical portion 27 and the circular-cylindrical grid 25. which have equal diameters.

FIG. 3 shows the shape of the fourth and the fifth grids not in accordance with the invention. The fourth grid 30 comprises two interconnected. circular-cylindrical portions 32 and 33. The fifth grid 31 is circulsbcylindrical and has the same diameter as the portion 33. The ends of the grids 30 and 3| facing each other are located in flat planes normal to the axis of the cylinders. The axes of the beams at the centers of the gap are indicated by 34, 35 and 36. which points are located on a line 37.

FIG. 4 is a sectional view at right angles to the axis of the converging lens of FIG. 3; FIG. 5 is a plan view of the converging lens of FIG. 3; FIG. 6 is a side elevation of the converging lens of FIG. 3.

In FIG. 4 the circular-cylindrical portion 33 of the grid 30 is shown with the three beams 38. 39 and 40 in the gap between the grids 30 and 31. the axes of which beams at this place are designated in FIG. 3 by 34, 35. 36 respectively.

FIG. 5 is the plan view. The intersections of a few equipotential planes formed by the gap due to the of a voltage difference between the grids 30 and 31 and the meridional plane going through the axes of the beams 38. 39 and 40 are designated by 41. 42. 43. 44. 45. 46 and 47. From the Figure it is apparent that on either side of the gap these intersection are symmetrical.

FIG. 6 is a side elevation. The intersections of few equipotential planes formed by the gap due to the application of the same voltage difference between the grids 30 and 3t and the plane going through the axis of the beam 38 at the center of the gap and normal to the meridional plane going through the axes of the beams 38. 39 and 40 are designated by 48. 49. 50. 51. 52. 53 and 54. From the Figure it will be apparent that these intersections are symmetrical on either side of the gap.

Because in the case described with reference to FIGS. 3. 4. 5 and 6 the axes of the beams 38 and 40 in the lens field between the fourth and fifth grids are located outside the axes of the cylinders. this lens field produces an aberration in the post-focusing of these beams. The axis of the gun in FIG. 7 is designated by 55. The cross-over of the beams 38 and 40 is reproduced virtually by the lens effect between the second and fourth grids approximately at the point 56 on the axis of the gun. These common cross-overs have to be reproduced by the lens effect between the fourth and fifth grids at one point on the screen. This lens effect therefore has to converge and focus the beams simultaneously. in fact said aberration is produced. which is evident from the path of the rays illustrated for the beams. The circle 57 is the section of the beam 38 taken in the main plane of the converging lens. The meridional rays of the beam located in the plane going through the axis 58 of the beam and the axis 55 of the gun the rays 59 and 60 thereof are illustrated in the Figure are focused approximately at point 6]. The rays located in the sagittal plane going through the axis of the beam the Figure illustrates the rays 62 and 63 are focused at point 64 on the axis 55 owing to the rotation-symmetry of the lens field to the axis 55. The meridional rays are thus more strongly focused towards the axis of the beam than the sagittal rays. in this manner two focal lines 65-66 and 67-68 are formed. which give rise to astigmatism of the electron spot obtained in the image space. The same applies to the beam 40. the section 69 of which taken in the main plane of the converging lens is shown. The centers of the circles 57 and 69 are located on a line 70. which intersects the axis 55 ofthe gun. The meridional rays 71 and 72 and the sagittal rays 73 and 74 of the beam 40 are shown in the Figure. The beam 39 (not shown) whose axis coincides with the axis 55 ofthe gun is focused atone point.

FIG. 8 illustrates a shape of the fourth and fifth grids according to the invention. The fourth grid 80 comprises two in tcrconnected. circular-cylindrical portions 82 and 83. The fifth grid 81 is circular-cylindrical and has the same diameter iiS the ortion 83. The ends of the grids 80 and 81 facing each other have a sinusoidal shape in a complementary manner so that the a has a constant width and the center of the gap has the same sinusoidal form. The axes of the beams at the centers of the gap are indicated by 84, 85. 86 and these points are located on a line 87.

FIG. 9 is a sectional view at right angles to the axis of the converging lens of FIG. 8; H0. is a plan view of the converging lens of FIG. 8; FIG. 11 is a side elevation of the converging lens of FIG. 8.

FIG. 9 shows the circular-cylindrical portion 83 of the grid 80 with the three beams 88. 89 and 90 at the gap between the rids 80 and 81. the axes of which beams at this place are designated in FIG. 8 by 84. 85 and 86 respectively.

H0. 10 is plan view. The intersections ofa few equipotenllili planes formed by the gap due to the application ofa voltage difference between the grids 80 and 8| and the meridional lane going through the axes of the beams 88. 89 and 90 are designated by 91.92. 93, 94. 95. 96 and 97. From the Figure it will be apparent that the intersections are substantially symmetrical on either side of the gap. As compared with the intersections 41 to 47 of FIG. 5 they are slightly curved at the area of the axes of the beams 88 and 90. the convex side facing the filth grid 81. Thus the focusing of the meridional rays of the beams 88 and 90 is slightly reduced.

FIG. 11 is a side elevation. The intersections of a few cquipotcntial planes formed by the gap due to the application of the voltage difference between the grids 80 and 81 and the plane going through the axis of the beam 88 at the center of the gap and normal to the meridional plane going through the axes of the beams 88. 89 and 90 are designated by 98, 99, I00, 101. I02. I03 and 104. From the Figure it will be apparent that the intersections are not symmetrical to the gap. As com pared with the intersections 48 to 54 of FIG. 6 they are curved at the area of the axis of the beam 88, the concave side facing the filth grid 81. Thus the focusing of the sagittal rays of the beam 88 is increased. in this manner the focusing of the meridional and sagittal rays of the beam 88 can be obtained at one point.

FIG. 12 is a developed view of part of the circular-cylindrical portion 83 of the grid and of part of the circular-cylindrical grid 81. The line of intersection with the plane containing the beams are designated by and 112. They intersect .the the edge of the portion 83 at points 106 and 108 as indicated in H0. 9. The lines of intersection with the plane normal to the plane of the beams passing through the axes of the grids are designated by 109 and Ill. They intersect the edge of the portion 83 at points 105 and'l07, as is shown in FIG. 9. The effective boundary line of the circular-cylindrical grids is the line 113 having the points of intersection I14. H5. H6 and 117 with the lines 109, I10, 111 and H2 respectively. The distances of the points and 117 from the display screen (not shown) are relative maxima and greater than the average distance of the line 113 from the display screen. it is thus ensured that in the beams 88 and 90 the meridional rays are focused to a lesser extent and the sagittal rays are focused to a greater extent so that the sagittal rays and the meridional rays are focused substantially at one point. Since the distances of the points 114 and 116 from the display screen are relative maxima and are equal to those of the points 115 and "7 from the display screen the disturbance of the rotation-symmetr caused by the configuration at 115 and 117 in the beams 89 located on the axes of the grids is substantially neutralized. in this case the field of the converging lens has four planes of symmetry and is quasi-symmetrical at the axis.

in a given case the circular openings 10. l! and t2 of the first grid 9 have a diameter of 0.75 mm, the circular openings l4. l5 and 16 of the second grid 13 have a diameter of 0.75 mm and the circular openings l8. l9 and 20 of the third grid 17 have a diameter of 2.0 mm. The distance of the line 22 through the centers of the openings l0. l4 and 18 from the axis 21 of the gun is 3.5 mm and the distance of the line 23 through the centers of the openings 12, 16 and :0 from the axis 2t of the gun is also 3.5 mm. The inner diameter of the portion 82 of the fourth grid 80 is 14 mm and that of the portion 83 is 20 mm. The inner diameter of the fifth grid 81 is also 20 mm. The dimension of the portion 82 in the direction of the gun axis is 7 mm, the average dimension of the portion 83 in said direction is 18 mm and the average dimension of the fifth grid 81 in said direction is ID mm. The distance between the portion 83 and the fifth grid 81 is 2 mm. The sinusoidal shape of the ends of the portion 83 and of the grid 81 has an amplitude of O.l8 mm. This gun may be driven by the following voltages:

cathode Oto I40 V first grid 0 V second grid L700 V third grid 350 V fourth grid 4.300 V lifth grid 25,000 V The variable cathode voltage serves for controlling the beam.

In this example. the beams are initially stlgmatic and the gap shape described involves a stigrnatic nature of the resultant electron spots. However, an electron spot of astigmatic nature may be desired. An example thereof is given by three beams located in a horizontal plane and scanning the display screen along horizontal lines. In given cases. for example for avoiding moire effects it may be desirable for the electron spots to have the shape of vertical lines. With initially stigmatic beams the grids of the converging lens then have the following shape. FIG. 13 shows a developed view of part of circular-cylindrical grids I20 and i2] located one on each side of a gap and having equal diameters. the grid 121 being nearest the display screen. The lines of intersection with the horizontal plane of the beams are designated by 123 and 125. The lines of intersection with the vertical plane going through the axes of the grids are designated by 122 and 124. The effective boundary line of the circular-cylindrical grids is the line 126 having the points of intersection I27. I28, 129 and 130 with the lines I22. I23. 124 and respectively. The distances of the points 128 and 130 from the display screen(not shown) are equal, are relative maxima and are greater than the average distance of the line 126 from the display screen. The distances of the points 127 and 129 from the display screen are equal, are greater than the distances of the points 128 and 130 from the display screen and are relative maxima. if the grids are straight profile, the meridional rays of the beams located outside the axes of the grids would be focused more strongly towards the axis of the beam than the sagittal rays so that the electron spot would have an astigmatic shape. Since the beams are located in a horizontal plane going through the axes of the cylinders, these beams would exhibit a vertical line in the plane of the focused meridional rays of these beams and these beams would exhibit a horizontal line in a plane further away from the gun and having the sagittal rays of these beams focused in it. Owing to the shape of the gap at 128 and 130 the difference in post-focusing of the meridional and sagittal rays of these beams is reduced. The situation of a vertical line in the plane in which the meridional rays are focused is maintained. however, and this plane is then the plane of the display screen. Owing to the shape of the gap at 128 and 130 the field on the axes of the grids is no longer rotation-symmetrical so that the electron spot of the central beam is astigmatic with a horizontal line in a plane in which the vertical rays are focused and a vertical line in a plane further away from the gun where the horizontal rays are focused. Since the distances of the points I27 and 129 from the display screen are relative maxima, the difference in post-focusing of the horizontal and vertical rays of this beam is reduced and since these distances are greater than those of the points 128 and 130 from the display screen. this difference is negative, which means that the beam has such an astigmatic nature that the plane in which the horizontal rays are focused and in which a vertical line is found is located nearer the gun than the plane in which the vertical rays are focused and in which a horizontal line is found. In this way it applies to the three beams that the venical line occurs in the plane nearest the gun.

FIG. 14 relates to a different configuration in accordance with the invention. This Figure is a developed view of part of circular-cylindrical grids 140 and 141 of equal diameters located one on each side of a gap, the grid 141 being located nearer the display screen. The lines of intersection with the horizontal plane of the beams are designated by 143 and 145. The lines of intersection with the vertical plane going through the axes of the grids are denoted by 142 and 144. The effective boundary line of the circular-cylindrical grids is the line 146, which has the points of intersection 147, 148, 149 and 150 with the lines 142, 143. 144 and 145 respectively. The

distances of the points 148 and from the display screen (not shown) are equal, are relative-maxima and are greater than the average distance of the line 146 from the display screen. The distances of the points 147 and 149 from the display screen are equal, are shorter than the distances of the points 148 and 150 from the display screen and are relative maxima. This configuration of the grids may be used for producing electron spots of stigmatic nature, while prior to passage through the lens formed by said grids the beams have such an astigmatic nature that their extension in a vertical direction exceeds that in the horizontal direction. This may be due to vertical, elliptic apertures in the pre-focusing grid so that an excessive pre-focusing of the rays located in a horizontal plane is produced.

What is claimed is:

1. A cathode ray tube comprising an evacuated envelope, an electron gun within said envelope for producing a plurality of electron beams positioned in a flat plane passing through the axis of said electron gun, at least two cylindrical electrodes arranged in consecutive order and surrounding the path of said beams, the axis of said two electrodes substantially lying in said flat plane and coinciding with the axis of one of said electron bearm, the ends of said electrodes confronting each other and defining an effective boundary line providing an aberration reducing electrostatic field for the electron beams not coinciding with the axes of said electrodes, said effective boundary line having a shape resulting from the combination of two sine curves, the period of one being twice the period of the other, a luminescent screen positioned within said envelope to intercept said electron beams, and a color selecting electrode within said envelope between said electron gun and said screen, said color selecting electrode being proximately spaced from and parallel to said screen, said cylindrical electrode substantially converging said beams in the plane of the color selecting electrode, said screen comprising a plurality of different luminescing substances, said electron gun producing a plurality of beams corresponding with the number of different luminescing substances, the distances between said effective boundary line and said screen having maximum values at areas of points of intersection of said effective boundary lines and said flat planes and planes orthogonal to said flat planes, respectively.

2. A cathode ray tube as claimed in claim 1 wherein said distance between said effective boundary line and said screen at areas of points of intersection between said effective boundary line and said flat plane is substantially equal to the distance at areas of points of intersection between said effective boundary line and said plane orthogonal to said flat plane.

0 0 O i I 

1. A cathode ray tube comprising an evacuated envelope, an electron gun within said envelope for producing a plurality of electron beams positioned in a flat plane passing through the axis of said electron gun, at least two cylindrical electrodes arranged in consecutive order and surrounding the path of said beams, the axis of said two electrodes substantially lying in said flat plane and coinciding with the axis of one of said electron beams, the ends of said electrodes confronting each other and defining an effective boundary line providing an aberration reducing electrostatic field for the electron beams not coinciding with the axes of said electrodes, said effective boundary line having a shape resulting from the combination of two sine curves, the period of one being twice the period of the other, a luminescent screen positioned within said envelope to intercept said electron beams, and a color selecting electrode within said envelope between said electron gun and said screen, said color selecting electrode being proximately spaced from and parallel to said screen, said cylindrical electrode substantially converging said beams in the plane of the color selecting electrode, said screen comprising a plurality of different luminescing substances, said electron gun producing a plurality of beams corresponding with the number of different luminescing substances, the distances between said effective boundary line and said screen having maximum values at areas of points of intersection of said effective boundary lines and said flat planes and planes orthogonal to said flat planes, respectively.
 2. A cathode ray tube as claimed in claim 1 wherein said distance between said effective boundary line and said screen at areas of points of intersection between said effective boundary line and said flat plane is substantially equal to the distance at areas of points of intersection between said effective boundary line and said plane orthogonal to said flat plane. 